Teacher's Guide for "The Southern Appalachians: A Changing World"
Mantle evolution recorded in Cr- spinel from the Bay of Island Ophiolitic Complex, northern...
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Transcript of Mantle evolution recorded in Cr- spinel from the Bay of Island Ophiolitic Complex, northern...
Mantle evolution recorded in Cr-spinel from the Bay of Island Ophiolitic Complex, northern
Appalachians.
1.University of Ottawa, Ottawa.2.Geological Survey of Canada, Québec.
Fabio Stern1, Keiko Hattori1 and Jean Bédard2
2
Introduction
Location and Geological Settings
Rock Types
Results
Conclusions
3
Cr-spinel (FeCr2O4) is a useful petrogenic indicator of mafic-ultramafic rocks since it is not easily altered.
Therefore we examined Cr-spinels and olivines from the ultramafic complex to evaluate the mantle evolution of the BOIC.
Introduction
4
The area is located on the west coast of Newfoundland.
It is part the western ultramafic belt of the northern
Appalachians.V
The Bay of Islands Ophiolite Complex is
exposed in four massifs:
• Tablelands
• North Arm
• Blow Me Down
• Lewis Hills
Location
Geological Settings
5
6
Harzburgite
1 m
opxnite
harzburgite
50cmdunite
7
0.5mm
Harzburgite
Ol
Ol
Opx
3mm
Chr
0.5mm
Chr
Chr
8
Dunite contains lenses and veins of coarse grained wehrlites.
Dunite
wehrlite
50 cm
9
Dunite
Ol
Chr
0.5mm
Ol Chr
Chr
Ol
10
Introduction
Location and Geological Settings
Rock Types
Results
Conclusions
11
• Harzburgites have high Ir-type PGEs (Ir, Os, Ru) typical of residual mantle rocks;
• Dunites have higher concentration of Pt and Pd, suggesting a cumulate origin.
Results
12
Chr
0.5mm
All harzburgite samples plot in the Ol-Sp mantle array (OSMA) defined by Shoji Arai (1994).
Fosterite component of olivine vs Cr# of spinel. Olivine-spinel mantle array. After Arai (1994).
spinelHarzburgite
Results
13
0.5mm
Dunites with Ol (Fo>90, ) plot in the OSMA;
Dunites outside the OSMA were products from fractionated melt. Not considered for the evaluation of the mantle.
Fosterite component of olivine vs Cr# of spinel. Olivine-spinel mantle array. After Arai (1994).
spinelDunite
Results
14
Chr
0.5mm
Large variation in the Cr#;
Change in mantle conditions from fertile to more refractory (Abyssal field to Forearc peridotite).
Fosterite component of olivine vs Cr# of spinel. Olivine-spinel mantle array. After Arai (1994).
Hz and Dun
Results
Max AbyssalPer.
15
Chr
0.5mm
Wide range from MORB to Boninitic fields
Results
Modified from Dare et al. (2008). Boninites data from Barnes & Roeder (2001).
Modified from Kamenetsky et al. (2001).
16
Chr
fO2 calc. based on Ballhaus (1991) Ol-Sp exchange thermometry.
Results:
•Hz shows low fO2;
•Dun shows higher fO2, suggesting the influence of subduction.
Asthenosphericmantle
Results
Modified from Wang et al. (2008).
17
Subduction initiation
18
Introduction
Location and Geological Settings
Rock Types
Results
Conclusions
19
Conclusions
• PGE abundance is effective in identifying its origin.
• Dunite in BOIC is a cumulate based on low Ir-type PGEs
• The mantle source had low S;
• The mantle changed gradually from abyssal peridotite-like to more
refractory conditions in subduction setting;
20
References
Arai, S., (1994). Characterization of spinel peridotites by olivine-spinel compositional relationships: review and interpretation. Chemical Geology 111: 191–204.
Ballhaus, C., Berry, R.F., Green, D.H., 1991. High pressure experimental calibration of the olivine-orthopyroxene-spinel oxygen geobarometer: implications for the oxidation state of the upper mantle. Contributions to Mineralogy and Petrology 107: 27–40.
Barnes, S.J., Roeder, P.L., (2001). The range of spinel compositions in terrestrial mafic and ultramafic rocks. Journal of Petrology 42: 2279–2302.
Dare, S., Pearce, J., McDonald, I., Styles, M., (2008). Tectonic discrimination of peridotites using fO2–Cr# and Ga–Ti–FeIII systematics in chrome–spinel. Chemical Geology, 261: 199-216.
Hattori, K., Wallis, S., Enami., Mizukami, T., (2009). Subduction of mantle wedge peridotites: Evidence from the Hgashi-akaiashi ultramafic body in the Sanbagawa metamorphic belt. Island Arc. 1-16.
Kamenetsky, V.S., Crawford, A.J., Meffre, S., (2001). Factors controlling chemistry of magmatic spinel: an empirical study of associated olivine, Cr-spinel and melt inclusions from primitive rocks. Journal of Petrology 42: 655–671.
McDonough, W.F., Sun, S.S. (1995): The composition of the earth. Chem. Geol.120: 223-253.
Nikolaeva, K., Gerya, T.V., Marques, F.O. (2010). Subduction initiation at passive margins: Numerical modeling. Journal of
Geophysical Research, 116: 1-19
Suhr, G. (1992): Upper mantle peridotites in the Bay of Islands Ophiolite, Newfoundland: formation during the final stages of a
spreading centre? Tectonophysics, 206: 31-53.
Wang, J., Hattori, K., Li, J., Stern., C. R., (2008). Oxidation state of Paleozoic subcontinental lithospheric mantle below the Pali Aike volcanic field in southernmost Patagonia. Lithos, 105: 98–110